Abstract : The gravitational wave detector VIRGO aims at extending the detection band down to a few Hertz by isolating the mirrors of the interferometer from seismic noise. This result is achieved by hanging each mirror through an elastic suspension (Superattenuator), designed to filter mechanical vibrations in all the degrees of freedom. An experimental upper limit of the mirror residual seismic noise at a few Hertz is provided in this paper. This is lower than the thermal noise floor, expected to limit the antenna sensitivity in the low frequency range.